child_process: optimize IPC for large data

Squashed from:
- child_process: stop indexOf() on whole IPC buffer
- child_process: get rid of forEach() and slice() in IPC
- child_process: get rid of another forEach() in IPC

Fixes: https://github.com/nodejs/node/issues/3145
PR-URL: https://github.com/nodejs/node/pull/10557


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>